Pontiac Fiero 1988 Service Manual page 493

6E2-A-38 DRIVEABILITY AND EMISSIONS - FUEL INJECTION (TBI)
ECM
12V
IGN.
15-WAY
I.P.
CONN.
-«-
H
DK GEM
389
W23
- — VSS SIGNAL
ELECTRONIC
I.P. CLUSTER
OR
f '
SPEEDOMETER
d z r
I
P/N SWITCH
BLK 450 —
ORN/BLK434
CLOSED IN
W18
PARK OR NEUTRAL
P/N SIGNAL
5-19-87
5-15-87
* 75 3940-6E
• CODE 24
VEHICLE SPEED SENSOR (VSS) CIRCUIT
2.5L "P" SERIES (TBI)
Circuit Description:
The E C M applies and monitors 12 volts on C K T 389. C K T 389 connects to the vehicle speed sensor which
alternately grounds C K T 389 when drive wheels are turning. This pulsing action takes place about 2000 times
per mile and the E C M will calculate vehicle speed based on the time between "pulses".
"Scan" reading should closely match with speedometer reading with drive wheels turning.
* * To prevent misdiagnosis, the technician should review E l e c t r i c a l Section "8A" or the E l e c t r i c a l
Troubleshooting Manual and identify the type of vehicle speed sensor used prior to using this chart. Disregard
a Code 24 set when drive wheels are not turning.
Test Description:
Numbers below refer to circled
numbers on the diagnostic chart.
1. Code 24 will set if vehicle speed equals 0 mph
when:
® Engine speed is between 1400 and 3600 rpm
• T P S is less than 2%
• Low load condition
• Not in park or neutral
• All conditions met for 5 seconds
These conditions are met d u r i n g a road load
deceleration. Disregard Code 24 that sets when
drive wheels are not turning.
2. 8-12 volts, at the I.P. connector, indicates C K T 389
is open between the I.P. connector and the V S S , or
there is a faulty vehicle speed sensor. A voltage of
less than 1 volt, at the I.P. connector, indicates
that C K T 389 wire is shorted to ground or open
between the connector and the E C M
The I. P. connector is located in the center console
near the E C M .
Diagnostic Aids:
"Scan" should indicate a vehicle speed whenever
the drive wheels are turning greater than 3 mph.
A faulty or misadjusted park/neutral switch can
result in a false Code 24. Use a "Scan" and check for
proper signal while in drive. Refer to C H A R T C-1A
for P/N switch diagnosis check.
If all O K , refer to "Intermittents" in Section " B " .